Different digital packet networks exist and are well known. There is hardly an office or campus without one or more local area packet networks. There are also higher layer networks that carry digital packets over successions of local and wide area networks, using some inter-networking protocol. The most ubiquitous and best known is the Internet. The invention relates to the transfer of real time signals over any and all of such digital communications networks.
The Internet and most local area networks are connectionless. Packets are routed and carried independently of each other to selected destinations on the basis of the destination address as given in the header of the packet. Also, as part of being connectionless, there is no reserved bandwidth for the carriage of packets between any particular source and destination pair, such carriage without reserved bandwidth being referred to as unresourced. Resourcing of packet carriage between any source and destination pair is feasible only if the carriage is along a particular path, i.e. is connection-oriented.
Connectionless local and wide area networks were developed for computer data communications which have no strict time constraints, and for which the networks are completely adequate and eminently suitable. Connectionless networks, and particularly the Internet, have come to be increasingly used also for real time communication, such as telephony and video conferencing for which such networks are, or will prove to be, manifestly inadequate. Given connectionless transfer without any access control, a network is exposed to the possibility of traffic overload. While the only effect of overloads is to slow down non-time sensitive data communications, they result in unacceptable delays in real time communications, causing disruption in the reception of the real time signal.
Disruption of real time communications by general network overloads can be reduced by providing a higher priority service in the network to them. But this can only be effective until the traffic at higher priority service in itself reaches overload proportions. To provide dependable and sustainable communications a network, including the Internet, needs to be connection-oriented with possibility of bandwidth reservation on connections, at least in regard to real time transfers.
Although bandwidth reservation is necessary for transfer of a real time signal, it is not of itself sufficient to assure timely transfer of the signal. Mere reservation of bandwidth would result in timely transfer if the signal had a constant rate, i.e. was carried in fixed size packets sent at regular intervals. Timely transfer is not possible if the signal has a variable rate—variable length packets, or packets at variable intervals, or both. To assure timeliness in the transfer of a variable rate signal, the network needs to forward packets in accordance with each packet's time requirements, on or before the deadline applicable to the packet. To accomplish that, network routers need to be time aware with respect to every packet on every established real time connection.